Six months pregnant, Elizabeth Kganyo was determined to cast her vote, even if it meant standing in a sun-baked queue for hours on end. “I was so excited because it was the first time,” she recalls, sitting outside her shack in Diepsloot, north of Johannesburg. “Everybody was happy.”

South Africa held its first multiracial election 20 years ago on Sunday (April 27), defying bombs, bluster and the threat of civil war to conjure a spectacle of voters in long, winding lines that ravished the world. But for Kganyo, like millions of others who put a cross beside the face of Nelson Mandela, those days of miracles and wonder are a fading memory. “It’s not the same now. We’re not happy to vote any more.”

Next month, South Africans return to the polls for the first election since Mandela’s death and the first in which the so-called “born free” generation — those whose lives began after racial apartheid — are eligible to vote. The African National Congress is in no doubt of a fifth consecutive victory on May 7 but faces an unprecedented long-term challenge both on the streets and at the ballot box.

Two decades of modest economic growth have left the white minority better off than ever but half of young black people without a job. South Africa is one of the most unequal societies on earth and reaping a whirlwind of frustration and unrest. One expression of this is the rise of a militant new party that claims to be the true inheritor of the ANC’s radical legacy.

The Economic Freedom Fighters (EFF) is bringing a flavour of Latin American socialism to an otherwise somewhat sterile election. It draws thousands of people to its rallies wearing an instantly recognisable motif, a red beret, as sported by the late Venezuelan president Hugo Chavez. Indeed, its “commander in chief”, charismatic firebrand Julius Malema, travelled to both Venezuela and Robert Mugabe’s Zimbabwe to hone his populist, anti-western rhetoric. The EFF regards forgotten places such as Diepsloot, a sprawling, impoverished and violent settlement, as fertile territory for recruiting angry young people who feel betrayed by the ANC.

Sprouting on farmland in the 90s, Diepsloot (“deep ditch” in Afrikaans) is a largely post-apartheid creation and most of its 200,000 residents blame the ANC for their dismal conditions. Amid this sea of shacks, piles of rubbish go uncollected and acrid water runs down unpaved dirt tracks.

Kganyo rises at 5:30am every day to commute to her suburban job as a domestic worker, earning 3,000 rand a month, while her husband is away for months at a time working down a mine. “Diepsloot is getting worse,” she said. “The price of taxis is up, the price of food is up. There are criminals here: sometimes you can’t go out at 10 o’clock.”

Kganyo has been on the waiting list for a government house for more than 10 years. “We always voted for the ANC but we’ve been living in the shacks for many years. They say they’re going to build a house and nothing happens.”

Surveys have found that the ANC’s political power rests on an underclass: at least two thirds of its voters are unemployed. Now it faces a competitor for this constituency for the first time. The EFF is hoping to invoke Mandela’s words from 1993: “If the ANC does to you what the apartheid government did to you, then you must do to the ANC what you did to the apartheid government.” The fledgling party essentially claims that Mandela’s revolution is half-finished: while political liberation was achieved 20 years ago, economic liberation remains elusive.

White-owned land is one emotive example. Soon after 1994, the ANC set a target of 25m hectares, representing 30pc of agricultural land, for transfer to black people within five years, but to date only about seven pc has been transferred.

The EFF’s manifesto includes wildly audacious promises to nationalise banks and mines, expropriate land without compensation, increase the minimum wage and double all social welfare grants. Critics say this would cost trillions of rand and bankrupt Africa’s wealthiest nation. They point to Zimbabwe, where seizures of farms wrecked agriculture and triggered catastrophic hyper-inflation.

But when pressed, Malema retorted: “Don’t talk like the Zimbabwean policy is the only one that has failed. Capitalism has failed under supervision of London and America. You have exploited Africa, you have created poverty in Africa.”

The EFF’s agenda sets it apart from a crowded election field in which the ANC’s principal opposition, the Democratic Alliance (DA), is even more pro-market and right of centre.

Indeed, Malema articulates the failures of the past 20 years with aplomb, picking at scabs and speaking painful truths. Expelled from the ANC for ill-discipline, he is now arguably its most eloquent critic. But Malema is a tainted figure in many eyes. He is facing court charges of fraud, corruption and tax evasion, and has been criticised for a lavish lifestyle that is at odds with his pro-poor declarations.

Malema, 33, will not be moving into the Union Buildings any time soon. The ANC’s formidable election machine looks set to take more than 60% of the vote. But with trade union solidarity behind it crumbling, and a new labour party mooted for the next election in 2019, some analysts believe the centre of gravity is shifting left.

The ANC insists that it has “a good story to tell” over the past 20 years. The government says it has built 3.7m houses, extended social welfare grants to 16 million people, doubled the number attending university and nurtured a new black middle class.

That is little consolation, however, to the people of Diepsloot, who feel the gap between haves and have-nots continues to widen.

Moeletsi Mbeki, a political economist, said: “The most critical time for African liberation movements is 15 to 20 years in power. This is where the critical moment happens because of lack of change in the economic system ... We have this black elite or middle class, call it what you may, who have been absorbed into the existing elite. That’s where the 15-year itch comes in because the masses start to say ‘these guys have promised all these things, but these things are not coming’.”

—By arrangement with the Guardian